That is not a "G" truck - they are a smaller SUV type vehicle with some having a small integrated bed or a camping compartment added by other companies. There are some 6x6 G-wagens but they are a relatively new development.
That truck in the photo is a Unimog 1300 type truck but is a 6x6 instead of the more typical 4x4 version. I am sure there is a different numeric designator for that configuration but its definitely a Unimog series truck.